Impaired production of blood cells is associated with morbidity and mortality. The classical model for blood cell replenishment is changing with the unexpected realization that hematopoietic stem cells (HSCs) directly sense and respond to pathogens via toll-like receptors (TLRs). Following TLR stimulation, murine HSCs proliferate and preferentially undergo myeloid-specific differentiation. Direct sensing of TLR ligand is thought to enable HSCs to immediately replenish innate immune cells that are rapidly depleted during acute infection. In contrast to the potential benefits of short-term HSC activation, chronic TLR stimulation dramatically impairs long-term HSC function. We have recently shown that murine HSCs chronically exposed to low-dose TLR4 agonist in vivo lose lymphoid potential, become exhausted, and fail to self renew. These findings are important because TLRs can be activated by endogenous fatty acids that are elevated in obesity as well as by plasma LPS or bacteria 16S rDNA which are present in patients with chronic infections. Like murine HSCs, in vitro studies show that human HSCs become activated and exhibit myeloid bias following TLR stimulation. However, the impact of TLR stimulation to human HSC function in vivo has not been established. Also unknown is the mechanism(s) by which chronic TLR stimulation perturbs HSC function. In Aim 1, we examine the impact of TLR4 stimulation to the self-renewal and multi-lineage reconstitution potential of human HSCs in vivo in a humanized mouse model. In Aim 2, we examine the mechanisms underlying TLR4-driven HSC skewing. These studies will be the first to establish the consequences of chronic TLR stimulation to human HSC competence in a pre-clinical model, and to establish the mechanism(s) by which TLR stimulation impairs HSC function.